To describe these mosaics, en cased in cupolas, apsides, squinches, pendentives, vaults and niches, as flat, or two-dimensional, would be inappropriate. True, there is no space behind the "picture-plane" of these mosaics. But there is space, the physical space enclosed by the niche, in front; and this space is included in the picture. The image is not separated from the beholder by the "imaginary glass pane" of the picture plane behind which an illusionistic picture begins: it opens into the real space in front, where the beholder lives and moves. His space and the space in which the holy persons exist and act are identical, just as the icon itself is magically identical with the holy person or the sacred event. The Byzantine church itself is the "picture-space" of the icons. It is the ideal iconostasis; it is itself, as a whole, an icon giving reality to the conception of the divine world order. Only in this medium which is common to the holy persons and to the beholder can the latter feel that he is himself witnessing the holy events and conversing with the holy persons. He is not cut off from them; he is bodily enclosed in the grand icon of the church; he is surrounded by the congregation of the saints and takes part in the events he sees. .
If, however, the icons were to exist in, and to share, a space which is normally the domain of the beholder, it was more than ever necessary to place them in individual receptacles, in spatial units which are, as it were, excrescences of the general space. Moreover, since the images are not links in a continuous chain of narrative, they must not flow into one another: they must be clearly separated and each must occupy its own place in the same manner as the events and persons they represent occupy distinct places in the hierarchical system. The formal means to this end is the separate framing of each single receptacle. The single units are set off either by their characteristic shapes as spatial units, especially in the upper parts of the building, or, in the lower parts, by being embedded separately in the quiet colour foil of the marble linings.